This invention relates to a device to control a continuously variable automatic transmission (CVT) which is particularly adequate for installation in a motor vehicle.
A CVT is a continuous transmission with a first bevel pulley pair upon an input shaft and a second bevel pulley pair upon an output shaft, each bevel pulley pair consisting of a first pulley stationary in the axial direction and a second bevel pulley movable in the axial direction (the primary pulley and secondary pulley, respectively). Between the bevel pulley pairs extends a belt drive organ such as a pushing linked band. The primary pulley and secondary pulley are adjusted by a pressure medium with the aid of an electronic control unit which controls the pressure level of the primary and secondary pulleys via electromagnetic actuators and hydraulic valves.
DE-OS 42 34 103 disclosed a device to control the pressure of the primary pulley. The device comprises an electromagnetically controlled primary valve and a hydraulic secondary valve. The primary valve is controlled by an electronic control unit. If the line from the electronic control unit of the primary valve is interrupted, the device changes over to emergency operation. The pressure level of the primary pulley is then determined by the secondary valve. The position of the secondary valve, in turn, is determined by the primary pulley via a mechanical clutch.
The hydraulic shifting circuit for such a CVT usually has one pump which, from a tank serving as a lubricant sump, conveys the pressure medium to the lines provided with pressure-reducing and pressure-regulating valves through which lines, among others, the adjusting cylinders for the primary pulley and secondary pulley are filled with pressure medium. The leakage flows which appear during adjustment of the primary and secondary pulleys pressurelessly flow back to the tank, the same as the leakage flows appearing in the pressure regulators, and are again returned by the pump, via a filter, to the hydraulic circuit. In the prior art, the known pressure regulators work in the range from 0 to 6 bar at an operating pressure of 8 bar.
The problem solved by the invention is based on increasing the efficiency and reducing the construction expenses of a hydraulic circuit for a CVT, in particular for a belt with bevel pulley transmission.
According to the invention, it is proposed that the hydraulic circuit be a closed hydraulic circuit with a constantly regulated pressure so that leakage flow currents from the primary pulley and secondary pulley, as well as the pressure regulators, no longer be delivered to the tank, but e.g. directly to the secondary circuit, or to a constant pressure system that supplies the hydrodynamic torque converter and the lubrication.